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Encoders

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Top 50 Contributor
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What is the difference between Absolute Encoders and Incremental Encoders? 

I was told that Incremental Encoders are exactly the same as Absolute Encoders except that you have to do a Homing operation at startup.  Beyond that I can't find anything about the various encoders on mounts.

What brand of encoders are used on the Taurus mounts?  Is it any of the following?

VIONiC

QUANTiC

TONiC

ATOM

RG2/RG4

SiGNUM

Top 50 Contributor
Posts 424
Ed33635 Posted: 07-17-2018 10:21 PM

What is the difference between Absolute Encoders and Incremental Encoders? 

I was told that Incremental Encoders are exactly the same as Absolute Encoders except that you have to do a Homing operation at startup.  Beyond that I can't find anything about the various encoders on mounts.

What brand of encoders are used on the Taurus mounts?  Is it any of the following?

VIONiC

QUANTiC

TONiC

ATOM

RG2/RG4

SiGNUM

Top 10 Contributor
Posts 2,420

Ed33635:
What is the difference between Absolute Encoders and Incremental Encoders? 
Absolute encoders know where they are at all times. Incremental encoders know only how far they have moved since they were initialized.

In the case of the Paramount, a crucial additional difference is that the absolute encoders are fitted directly to the mount axes themselves, bypassing the worm drives. The incremental encoders are fitted to the worms instead, which means that with incremental encoders alone periodic errors in the worm drive show up in the tracking. These have to be calibrated and corrected using PEC, the assumption being made that they are stable over time and not dependent on pointing direction.

Ed33635:
I was told that Incremental Encoders are exactly the same as Absolute Encoders except that you have to do a Homing operation at startup.
"Exactly the same" is misleading. What the homing operation does is re-zero the incremental encoders when precise sensors on each of the axes are reached. From then on, the incremental encoders are in effect delivering absolute readings. However, the crucial additional difference remains, namely that the encoder system is behind the worm drive and you will need PEC to get the best out of the RA tracking.

Ed33635:
What brand of encoders are used on the Taurus mounts?
Someone from SB should answer this, but last time I looked they were using Renishaw encoders.

Notes:

  • Absolute encoders of a given accuracy are much more expensive than the incremental equivalent.
  • Modern large telescopes invariably use on-axis absolute encoders.
  • Some large telescopes a few decades ago had both absolute and incremental encoders, absolute for blind pointing and incremental for tracking. For example the 3.9m AAT had 0.05 arcsec resolution incremental encoders supplemented by absolute encoders of about 1 arcsec resolution. And because the incremental encoders were fitted to a gear in the drive system rather than being on-axis, it had periodic error just like a Paramount does:

 

Top 50 Contributor
Posts 424

Thank you.  That explains it very well.

Ed

 

Top 10 Contributor
Posts 3,324
Software Bisque Employee

Ed33635:
What is the difference between Absolute Encoders and Incremental Encoders?

Incremental encoders must be aligned to an origin in some fashion, continually monitored for movement, and the total number of "counts" passed per movement tallied in order to determine the present position. If the movement is too rapid, the position can be lost due to skipped "counts" and ambiguity about the direction of movement.

Absolute encoders produce a unique digital pattern for every position (to their rated resolution), thus avoiding the above requirements and problems.

All Paramount mounts are equipped with an incremental rotary shaft encoder on the motor for each axis. They are also equipped with a homing feature that allows them to perform essentially like absolute encoders. This requires that the mount first be homed when powered on; the mount automatically seeks the home position, after which the control system knows the true orientation of the mount. Rotating a mount axis by hand (rather than motor) is not observed by the encoder.

Some Paramount models (the Taurus 500 and 600) are also equipped with—or (the ME II and Taurus 400) can be retrofitted with—absolute on-axis encoders. When so equipped, homing is not required, as the control system always knows the mount's orientation—immediately when powered on and regardless of how fast or by what method the mount moves. And having on-axis encoders provides other benefits, as is described for the Paramount ME II (but applies to Taurus as well) here.

You can read more about the differences between incremental and absolute encoders in this Wikipedia article.

Ed33635:
What brand of encoders are used on the Taurus mounts?

Why do you ask?

Brian S. Rickard
Software Bisque, Inc.

Top 50 Contributor
Posts 424

Thanks Brian,

I asked about the brand of encoder because I thought there might be a difference in accuracy between brands but in communicating with Renishaw I found out that all their encoders are of basically equal accuracy.

Ed

 

Top 10 Contributor
Posts 3,324
Software Bisque Employee

You're welcome.

For the benefit of other readers, I've merged here your identical question—and Mr. Wallace's answer—from our Paramount ME Forum.

Brian S. Rickard
Software Bisque, Inc.

Top 100 Contributor
Posts 195

Hi Brian,

Is it possible and would it be beneficial to add an absolute encoder to just the RA axis to improve tracking, while saving $3,000?

Thanks

John

Top 10 Contributor
Posts 3,324
Software Bisque Employee

I don't know if that's an original idea, but I don't recall it being asked before. I'm not the best to answer this (and I'll check with those who are), but while I think it would theoretically be possible and beneficial, I'm pretty sure implementing this would require some significant software (and possibly additional firmware) development time, and our developers already have full plates. Of course, homing would still be required for the declination axis.

Brian S. Rickard
Software Bisque, Inc.

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